In mobile communication systems, introduction of device-to-device (D2D) communication, in which a radio terminal directly communicates with another radio terminal, has been discussed. For example, 3rd Generation Partnership Project (3GPP), which defines standard specifications of mobile communication systems, specifies Proximity-based services (ProSe) as the D2D communication in Non-Patent Literature 1. ProSe includes ProSe discovery and ProSe direct communication. ProSe discovery makes it possible to detect proximity of radio terminals. ProSe direct communication enables establishment of a communication path between radio terminals discovered by the ProSe discovery.
Patent Literature 1 discloses a discovery procedure between radio terminals that perform D2D communication. Specifically, a User Equipment (UE) 100-1 transmits a discovery signal by broadcasting and a UE 100-2 performs processing for receiving the discovery signal that has been transmitted. The UE 100-2 performs processing for receiving the discovery signal, to thereby discover the UE 100-1 that has transmitted the discovery signal. Further, the UE 100-2 transmits a response signal to the UE 100-1, whereby the UE 100-1 is able to determine that it has been discovered by the UE 100-2. The UE 100-2 determines in advance regarding whether it is capable of performing D2D communication with the UE 100-1 based on the distance between the UE 100-2 and the UE 100-1. Therefore, the UE 100-2 is able to perform processing for receiving the discovery signal that has been transmitted from the predetermined UE in advance.